Adhesives have found use in a wide variety of industrial applications especially in bonding substrates together. New areas of possible application exist for coupling together, e.g. joining, two or more parts whose dimensions may not always exhibit a perfect fit. Such mismatched structures are frequently encountered in metal stamping operations because of differences in the spring back characteristics of the particular sheet metal used. Another example of such an imperfect fit is encountered in a tubular joint connection consisting of a female member and a male member, which is a common part of the piping system of many household appliances. Currently, this type of joint is assembled by inserting the male member into the female member and fastening the assembly together with a metal fastener or clip. A problem arises where in a typical production operation there are variations statistically produced in the diameters of both the female and male numbers and therefore the fastening together of the assembly by means of a mechanical fastener does not produce an exact fit. This results in a potential leak in the system due to the formed gap between the surfaces and is obviously detrimental to the working efficiency of the entire system, particularly when the tube is part of a piping system conducting hot or cold water or a refrigerant.
Thus, a need exists for an adhesive which possesses good adhesion to untreated and treated metal, as well as good gapfilling properties between the surfaces of the mismatched parts, to insure satisfactory mechanical performance of the assembly. Also, the adhesive should possess the proper tack to station the two elements together upon assembly and a high green strength to maintain that position and spatial integrity of the two elements as the assembly is prepared for the subsequent curing step. Also, the adhesive should have a relatively long pot life before curing and reasonably fluid viscosity to enable it to be applied in the molten state by a suitable applicator.
Hot melt adhesives have been found to be unsatisfactory mainly because of a large contraction upon cooling. As a result, high stresses develop in the adhesive layer which on cooling leads to failure of the joint. Thermosetting materials have been found to possess lower than adequate green strength to hold the male member firmly in the female member before curing, resulting in a dislodging of the male member by applied stress before curing. Blends of thermoplastic and thermosetting materials have been used employing mainly copolyesters as the thermoplastic component and an epoxy resin as the thermosetting component. The blends containing a relatively low molecular weight epoxy resin give good tack and adhesion properties, but poor green strength. On the other hand, blends containing a high molecular weight epoxy resin given good green strength but poor tack and initial adhesion upon joining the elements together. Blend of such thermoplastic and thermosetting materials that are used are typical of those which are known in the prior art and are described in U.S. Pat. No. 3,723,528 and 3,723,529 (Hoeschele), U.S. Pat. No. 3,749,758 (Gannon) and U.S. Pat. No. 3,329,740 (Battersby).